Optical simulation applications such as aircraft flight simulation often employ an optical probe which is moved over a scale model of terrain at a variable altitude and which transmits an image of the terrain model to a sensor, e.g. a vidicon camera tube. Optical probes of the Scheimpflug type are particularly effective in this application since they provide erect images of the terrain model with near infinite depth of field. Such a probe has three basic elements: an objective for viewing an object plane and forming an image of the object; a tilt-focus relay lens for receiving that image; and a sensor for receiving the projected image relayed by the tilt-focus relay lens. These elements are arranged to satisfy the Scheimpflug condition which states that the plane containing the original object, the plane containing the projected image, and the principal plane of the objective lens all intersect along one common line defined by Scheimpflug as the axis of colineation. (See U.S. Pat. No. 751,347 issued Feb. 2, 1904.) Optical scanning probes of this type are described in "Optical Scanning Probe Technology (Tutorial)" by Martin Shenker, Procedings of the Society of Photo-Optical Instrumentation Engineers, Simulators & Simulation, vol. 59, pp. 71- 79 March, 1975.
The tilt-focus relay lenses used in these optical probes are adjustably inclinable relative to the optical axis of the probe. Experience in the design of such tilt-focus lenses has demonstrated that there is a limit to the tilt angle that can be achieved with these lenses with acceptable spherical aberration. Although the tangential oblique spherical aberration can be controlled or corrected, the sagittal oblique spherical aberration was essentially unalterable-- remaining as a significant over-correction, increasing rapidly with increasing tilt angle.
In the course of studying this problem of spherical aberration in tilt-focus relay lenses, I discovered that if the controllable tangential oblique spherical aberration could be made to match the sagittal oblique aberration of such a lens, then the amount of spherical aberration could be corrected by additional axial components within the probe, and these additional components could introduce a variable amount of spherical aberration into the system-- the amount being variable with the tilt of the tilt-focus lens. My present invention provides a simple solution to this problem, one embodiment of that invention comprising only four lenses in an afocal system.